Bitumen is a heavy type of crude oil that is often found in naturally occurring geological materials such as tar sands, black shales, coal formations, and weathered hydrocarbon formations contained in sandstones and carbonates. Some bitumen can be described as flammable brown or black mixtures or tarlike hydrocarbons derived naturally or by distillation from petroleum. Some bitumen can be in the form of a viscous oil to a brittle solid, including asphalt, tars, and natural mineral waxes. Substances containing bitumen are typically referred to as bituminous, e.g., bituminous coal, bituminous tar, or bituminous pitch. At room temperature, the flowability of some bitumen is much like cold molasses. Bitumen can be processed to yield oil and other commercially useful products, primarily by cracking the bitumen into lighter hydrocarbon material.
As noted above, tar sands represent one of the well known sources of bitumen. Tar sands typically include bitumen, water and mineral solids. The mineral solids can include inorganic solids such as coal, sand, and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it can also be buried two thousand feet below the surface overburden or more. Tar sands deposits are measured in barrels equivalent of oil. It is estimated that the Athabasca tar sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves are estimated to be about 1.3 trillion barrels.
The bitumen content of some tar sands varies from approximately 3 wt % to 21 wt %, with a typical content of approximately 12 wt %. Accordingly, an initial step in deriving oil and other commercially useful products from bitumen typically requires extracting bitumen from the naturally occurring geological material. In the case of tar sands, this can include separating the bitumen from the mineral solids and other components of tar sands.
One conventional process for separating bitumen from mineral solids and other components of tar sands includes mixing the tar sands with hot water and, optionally, a process aid such as caustic soda (see, e.g., U.S. Pat. No. 1,791,797). Agitation of this mixture releases bitumen particles from the tar sands and allows air bubbles to attach to the released bitumen particles. These air bubbles float to the top of the mixture and form a bitumen-enriched froth. In Applicant's experience, such a froth typically includes around 60% bitumen, 30% water, and 10% inorganic minerals. The bitumen-enriched froth is separated from the mixture, sometimes with the aid of a solvent, and further processed to isolate the bitumen product. For example, the froth can be treated with an aliphatic (pentane-type) or an aromatic (naphtha-type) solvent to produce a clean bitumen product that can serve as a refinery upgrader feed stock. The bulk of the mineral solids can also be removed to form a tailings stream. Typically, the tailings stream also includes water, solvent, precipitated asphaltenes (in the case where the asphaltene is not soluble in the solvent used to separate the bitumen-enriched froth from the mixture), and some residual bitumen.
One issue with conventional hot water extraction methods is that they may achieve relatively low bitumen recoveries when used on low grade bituminous materials (e.g., bituminous material having a bitumen content of 10 wt % or less). Low recovery rates will be especially problematic in regions where a regulatory board stipulates a minimum bitumen recovery for certain grades of bituminous material. For example, the Alberta Energy and Utilities Board has implemented guidelines requiring that the bitumen recovery rate for hot water extraction using naphtha-based froth treatment of ore sands having less than 11% bitumen content satisfy the following equation:Bitumen Recovery≧−2.5*(Ore Grade)2+54.1*(Ore Grade)−202.6  (1)where both Bitumen Recovery and Ore Grade are expressed in wt-%. FIG. 1 provides a graphical representation of Equation (1) set against data for actual bitumen recoveries achieved on ore sands of various grades when using a hot water extraction process using paraffinic froth treatment. In FIG. 1, the line marked A represents the minimum required bitumen recovery rate for various ore grades as defined by the Alberta Energy and Utilities Board according to Equation (1) above, while line B represents a least square fit of a set of extraction data from a hot water extraction operating plant in the Athabasca region. As can be seen in FIG. 1, the actual bitumen recoveries achieved from this set of data in the range between approximately 10% and 11% fall below the board's directive for bitumen extraction. Furthermore, an extrapolation of line B back towards the lower ore grades would suggest that the actual bitumen recoveries from the hot water extraction method continue to fall below the mandated minimum. Accordingly, it is possible that these hot water extraction methods will not be permitted for extracting bitumen from lower grade bituminous material.
The difficulty for conventional hot water extraction methods in extracting bitumen from low grade bituminous materials typically stems from the impact that hot water has on the relatively high content of certain clay components in low grade tar sands ores. In Applicant's experience, the introduction of caustic hot water during the extraction process typically causes certain clay components (e.g., montmorillonite) in the bituminous material to activate and swell, especially when the caustic hot water contains divalent ions such as calcium. The swollen and activated clay will then mix with the water phase introduced to the bituminous material by the hot water extraction methods and produce a clay suspension with a relatively high viscosity and density. If the clay suspension is present rather than just hot water, surfactants produced during the natural weathering of the asphaltene components of the bitumen phase that normally liberate bitumen by reducing interfacial tensions between bitumen particles and sand particles will instead absorb on the clay particles. A reduction in the number of liberated bitumen particles will likely impact the efficient production of a high grade bitumen froth, as there are fewer liberated bitumen particles to attach to air bubbles during the flotation step. Furthermore, it is typically more difficult for proper air bubbles to be formed in a clay suspension.
As a result of the inability of conventional hot water extraction methods to recover acceptable amounts of bitumen from low grade bituminous materials, the versatility of the conventional methods is curtailed. The conventional methods are limited to processing higher grade bituminous materials, which ultimately makes the conventional methods more expensive to carry out. Additionally, without a method for economically processing low grade bituminous material, a significant portion of the world's bitumen resources can end up going to waste.